Distribution of Arsenic and Iron in Hyporheic Zone Sediments Along the Hooghly River

Arsenic (As) contamination within the shallow aquifers of the Bengal basin continues to pose a serious health risk to millions of people who rely on the groundwater for drinking purposes. Elevated dissolved As concentrations in the aquifers are attributed to the reductive dissolution of As-bearing Fe-oxides. Within the hyporheic zone (HZ), interactions between oxygen-rich river water and reducing groundwater causes the precipitation of Fe-oxides, which act as a sink of As. Surficial fine sediment has been proposed to limit this reaction. Once formed, the As-bearing Fe-oxides may dissolve under reducing conditions to further contaminate the adjacent aquifer. In this preliminary study, sediments from silt-capped and sandy riverbanks along the Hooghly River (West Bengal, India) were investigated to understand the factors controlling the mobility of As within the HZ. Bulk elemental concentrations were measured by X-Ray Fluorescence and the relative proportion of Fe(III) was estimated by diffuse reflectance spectroscopy. The silt-capped riverbanks had As and Fe concentrations of 3.6 mg/kg and 19.0 g/kg, respectively, which were more closely associated with clay minerals as shown by the ΔR which is a proxy for Fe(III) (ΔR at 520 nm = 0.2). The sands had As and Fe concentrations of 3.6 mg/kg and 12.5 g/kg, respectively, with higher proportions of Fe present as Fe-oxides (ΔR at 520 nm = 0.37). The results indicate that the distribution of As and Fe differs between the sandy and silt-capped riverbanks, indicating that the hydrological and chemical reactions impacting As mobility varies between the riverbanks.